The electronics and microelectronics industries rely on chemical mechanical planarization techniques for preparing sophisticated electronic devices that pervade our modern world. These include microprocessors and other integrated circuits that rely on silicon and other semiconductor materials; solid state and hard disk memory devices; optical materials and devices; and various other commercial and consumer electronic items.
One ubiquitous example is the hard magnetic disk, used for storing digital information in a manner that allows for highly efficient random access of the information. Hard disks now available for memory applications include multiple layers of different materials coated onto a rigid disk base. Each layer can have a different specialized function based on specific mechanical or magnetic properties of the material of the layer. One of the layers functions as a magnetic storage layer. But the other layers are also critical to reliable performance of the hard disk product, meaning that precision placement of each and every layer is essential in preparing the disk.
One layer that is common in a hard memory disk is a non-magnetic nickel layer, e.g., NiP, especially electroless NiP, which is present to provide hardness. The processed nickel layer is strong, hard, and can be processed to a highly smooth and uniform surface that allows the nickel layer to serve as a base for added magnetic and other layers. See, e.g., U.S. Pat. No. 6,977,030. The nickel layer can be provided by electroless plating and may be referred to as “electroless” NiP. See, e.g., U.S. Pat. No. 6,410,104.
The nickel layer as applied will have surface properties (roughness, micro-waviness, flatness) that are not immediately suitable for further processing into a hard disk, such as by applying additional constituent layers of the multi-layer hard disk. Current manufacturing techniques include processing the surface of an applied nickel layer by chemical mechanical processing to improve the surface properties, especially smoothness (i.e., reduced roughness and micro-waviness), before depositing additional layers of a hard disk product.
In processing the surface, various CMP techniques, abrasive particles, and chemicals have been used. Important factors in methods for preparing the nickel layer surface include a relatively high removal rate (to maintain processing throughput and an adequate cost of operation) and highly uniform resultant surface properties with a low level of scratching.
Balancing factors of removal rate and highly refined surface properties leads to different processing options, such as using relatively hard abrasive particles to provide higher removal rates, but which can produce excessive scratching, versus using softer particles that provide less scratching but have a lower removal rate. Sometimes, multiple separate steps are used including a first step that uses hard abrasive particles to affect a high removal rate, followed by at least one subsequent step using softer particles that have a lower removal rate but provide a more gentle (fine) process and a final, low roughness, low scratch surface. The first step may include the use of one or more of: hard abrasive particles (e.g., alumina particles), relatively large abrasive particle size, a relatively hard CMP pad, and relatively high pressure between the pad and the nickel layer surface during processing. In a subsequent (fine) step designed to provide a finished (low roughness) surface at a relatively lower removal rate, generally smaller sized and softer (e.g., silica) particles may be used along with lower pad pressure and a relatively softer CMP pad. Ultimately the nickel layer surface exhibits a low roughness, low micro-waviness, and a low level of scratching, to which subsequent layers may be applied.
Many slurries used for processing a nickel phosphorus surface contain alumina or a mixture of alumina and colloidal silica as abrasive. Alumina is more typically used for achieving a relatively high removal rate, such as in a first or early step. But due to the hardness of alumina, these abrasive particles can become embedded in a surface of the nickel layer during processing, forming an embedded (alumina) particle defect. The alumina abrasive particle can remain embedded in the nickel layer throughout subsequent processing, eventually forming a surface protrusion in a finished hard magnetic disk surface. During use of the hard magnetic disk product, the surface protrusion can directly cause a head crash as a magnetoelectric read or write head flies over the magnetic disk surface, contacting the surface protrusion.
One way to eliminate embedded particle defects and their associated potential for causing a head crash is to use CMP processes and slurries that have a reduced need for alumina particles, by developing abrasive slurries that contain a reduced amount of alumina abrasive particles, preferably even slurries that contain no alumina particles (i.e., “alumina-free” slurries). Yet high manufacturing throughput and commercially-feasible cost of ownership requirements ensure that high removal rate remains a high priority in processing a nickel layer of a hard disk. Slurries that contain only silica as abrasive particles (“silica-only” slurries) have been studied as one possible solution to embedded particle defects. At present, however, removal rates (of NiP) using slurries that contain only silica abrasive particles remain low compared to alumina-containing slurries.